Optical transceiver and method for controlling optical transceiver

ABSTRACT

An optical transceiver is configured to receive an optical signal on which a monitoring signal encoded by Manchester encoding is superimposed. The optical transceiver includes a decoding circuit configured to decode the monitoring signal from an electrical signal generated from the optical signal. The decoding circuit includes an interval measuring unit and a decoding unit. The interval measuring unit is configured to detect only a rising edge or a falling edge of a waveform of the electrical signal, to measure a first time interval between a detected first edge and a second edge detected immediately after detecting the first edge, and to measure a second time interval between the second edge and a third edge detected immediately after detecting the second edge. The decoding unit is configured to decode the monitoring signal encoded by the Manchester encoding based on a ratio between the first and second time intervals.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority to JapanesePatent Application No. 2020-194066 filed on Nov. 24, 2020, the entirecontents of which are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to an optical transceiver and a methodfor controlling an optical transceiver.

2. Description of the Related Art

Manchester code is a code that assigns each of two logical values to betransmitted to a rising edge and a falling edge of a signal waveform.For example, a receiver that generates a code value and a synchronizedclock from the Manchester code data measures the time of the edgeinterval of either the rising edge or the falling edge of the receivedsignal. The receiver determines whether the edge of the time is a bitmiddle point or a bit end point from the measured time and generates acode value and a synchronized clock, as described in Japanese PatentApplication Laid-Open No. 2005-142615.

However, the receiver that decodes the Manchester code described abovecannot generate a code value unless the receiver knows the bit widththat is the period of the synchronized clock included in the Manchestercode data. Therefore, when the bit width varies due to changes intemperature of the receiver or changes over time, and the like, andfurther when the bit width exceeds an allowable error of variation,there is a concern that the code value of the Manchester code cannot becorrectly decoded.

The Manchester code has an ambiguity in which a pattern having the sameconsecutive code values and a pattern having opposite consecutive codevalues cannot be distinguished from each other. For this reason, whenManchester code is attempted to be received from midstream, a code valueopposite to the expected code value may be decoded.

Thus, the present disclosure is intended to provide an opticaltransceiver capable of decoding a code value of Manchester code evenwhen a period of a synchronized clock used in Manchester encoding isunknown or when the period of the synchronized clock varies.

SUMMARY OF THE INVENTION

An optical transceiver according to an embodiment of the presentdisclosure is configured to receive an optical signal on which amonitoring signal encoded by Manchester encoding is superimposed, andincludes a decoding circuit configured to decode the monitoring signalfrom an electrical signal generated from the optical signal. Thedecoding circuit includes an interval measuring unit and a decodingunit. The decoding circuit is configured to detect only a rising edge ora falling edge of a waveform of the electrical signal, to measure afirst time interval between a detected first edge and a second edgedetected immediately after detecting the first edge, and to measure asecond time interval between the second edge and a third edge detectedimmediately after detecting the second edge. The decoding unit isconfigured to decode the monitoring signal encoded by the Manchesterencoding based on a ratio between the first time interval and the secondtime interval.

Additional objects and advantages of the embodiments are set forth inpart in the description which follows, and in part will become obviousfrom the description, or may be learned by practice of the disclosure.The objects and advantages of the disclosure will be realized andattained by means of the elements and combinations particularly pointedout in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory and are not restrictive of the disclosure asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a circuitconfiguration of an optical transceiver according to an embodiment;

FIG. 2 is a signal waveform diagram illustrating an example of arelationship between Manchester code and an edge interval;

FIG. 3 is an explanatory diagram illustrating an example of a ratiorepresenting a change in two consecutive edge intervals in Manchestercode;

FIG. 4 is an explanatory diagram illustrating an example of a method ofestimating an edge interval according to a ratio between two consecutiveedge intervals of slowly received data signals;

FIG. 5 is an explanatory diagram illustrating an example of a method ofdecoding a code value based on an edge interval of a slowly receiveddata signal and a code value decoded immediately before;

FIG. 6 is a circuit diagram illustrating an example of a single-edgeinterval counter of FIG. 1 ;

FIG. 7 is a circuit diagram showing an example of a decoding unit ofFIG. 1 ; and

FIG. 8 is a state transition diagram illustrating an example of anoperation of a Manchester decoding circuit of FIG. 1 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Description ofEmbodiments of the Present Disclosure

First, embodiments of the present disclosure will be described bylisting.

[1] An optical transceiver according to an embodiment of the presentdisclosure is configured to receive an optical signal on which amonitoring signal encoded by Manchester encoding is superimposed, andincludes a decoding circuit configured to decode the monitoring signalfrom an electrical signal generated from the optical signal, thedecoding circuit including an interval measuring unit configured todetect only a rising edge or a falling edge of a waveform of theelectrical signal, to measure a first time interval between a detectedfirst edge and a second edge detected immediately after detecting thefirst edge, and to measure a second time interval between the secondedge and a third edge detected immediately after detecting the secondedge, and a decoding unit configured to decode the monitoring signalencoded by the Manchester encoding based on a ratio between the firsttime interval and the second time interval. This allows the opticaltransceiver to decode the monitoring signal encoded by the Manchestercoding based on the ratio between consecutive first and second timeintervals, even when a period of a synchronized clock used in theManchester encoding is unknown or even when the period of thesynchronized clock varies.

[2] In the above [1], the decoding unit may include a clock numberestimating unit configured to estimate a number of clocks of the secondtime interval with respect to a period of a synchronized clock used inthe Manchester encoding of the monitoring signal based on the ratiobetween the first time interval and the second time interval, and a codevalue determining unit configured to decode the monitoring signalencoded by the Manchester encoding based on the estimated number ofclocks of the second time interval and a logical value of the monitoringsignal decoded immediately before. By measuring the first and secondtime intervals and determining the number of clocks of the second timeinterval from the ratio of the time interval, the monitoring signalencoded by the Manchester encoding can be decoded even when the periodof the synchronized clock is unknown. In addition, the bit width that isthe period of the synchronized clock can be determined from the numberof clocks in the second time interval.

[3] In the above [2], the clock number estimating unit may include astorage unit configured to store the first time interval, a plurality ofmultipliers multiplying the first time interval by each of a pluralityof thresholds, and a plurality of comparators configured to compare eachof a plurality of multiplication results from the plurality ofmultipliers to the second time interval, wherein the clock numberestimating unit may estimate the number of clocks based on a pluralityof comparative results from the plurality of comparators. This allowsthe number of clocks in the second time interval to be estimated and themonitoring signal to be decoded by performing multiplication by amultiplier instead of division to obtain the ratio between theconsecutive first time interval and the second time interval. As aresult, the circuit size of the decoding unit can be inhibited, and thecost of the optical transceiver can be reduced.

[4] In [1], the interval measuring unit may further measure a third timeinterval between the third edge and a fourth edge detected immediatelyafter detecting the third edge, and the decoding unit may be configuredto decode a first logical value corresponding to the third edge of themonitoring signal based on the ratio between the first time interval andthe second time interval and to decode a second logical valuecorresponding to the fourth edge of the monitoring signal based on theratio between the second time interval and the third time interval.Thus, the decoding unit can sequentially determine consecutive codevalues of the Manchester code using consecutive pairs of two timeintervals.

[5] In [1], upon detecting by the decoding unit that the ratio betweenthe first time interval and the second time interval is out of apredetermined range, the interval measuring unit may further measure athird time interval between the third edge and a fourth edge detectedimmediately after detecting the third edge, and a fourth time intervalbetween the fourth edge and a fifth edge detected immediately afterdetecting the fourth edge, and the decoding unit may decode themonitoring signal based on the ratio between the third time interval andthe fourth time interval. This allows the decoding unit to resumedecoding of the monitoring signal encoded by the Manchester encodingagain without continuing to detect an incorrect code value.

[6] In [1], the monitoring signal encoded by the Manchester encoding andreceived by the decoding circuit may include a correction pattern inwhich a first logical value, a first logical value, a second logicalvalue, and a first logical value sequentially appear, and the firstlogical value may be indicated by a rising edge or a falling edgedetected by the interval measuring unit. Therefore, for example, whenthe code of the first consecutive logical value continues to beerroneously detected as the code of the second consecutive logicalvalue, the decoding unit can return the received operation of the codevalue to normal by receiving the correction pattern and can detect thecorrect code value thereafter.

[7] A method for controlling an optical transceiver according to anotherembodiment of the present disclosure is a method for controlling anoptical transceiver configured to receive an optical signal on which amonitoring signal encoded by Manchester encoding is superimposed and todecode the monitoring signal from an electrical signal generated fromthe optical signal, and in the method, only a rising edge or a fallingedge of a waveform of the electrical signal is detected; a first timeinterval between a detected first edge and a second edge detectedimmediately after detecting the first edge is measured; a second timeinterval between the second edge and a third edge detected immediatelyafter detecting the second edge is measured; and the monitoring signalencoded by the Manchester encoding based on a ratio between the firsttime interval and the second time interval is decoded. This allows theoptical transceiver to decode the monitoring signal encoded by theManchester encoding based on the ratio between consecutive first andsecond time intervals, even when the period of the synchronized clockused in the Manchester encoding is unknown or even when the period ofthe synchronized clock varies.

Details of Embodiments of the Present Disclosure

An embodiment of an optical transceiver of the present disclosure willbe described with reference to the drawings. In the followingdescription, identical symbols are provided for identical elements orcorresponding elements, and the description thereof may be omitted.Symbols of the input terminal, the output terminal, and each node arealso used as symbols of signals.

Embodiment

[Optical Transceiver Circuit Configuration]

FIG. 1 is a block diagram illustrating an example of a circuitconfiguration of an optical transceiver according to an embodiment. Forexample, an optical transceiver 100 illustrated in FIG. 1 is connectedto a communication device, such as a host device, which transmits andreceives optical signals. The optical transceiver 100 has a function ofconverting a digital signal into an optical signal and transmitting theoptical signal to another optical transceiver, and converting theoptical signal received from another optical transceiver into a digitalsignal. The optical transceiver 100 includes a DSP (Digital SignalProcessor) 10, a Manchester encoding circuit 20, a photoelectricconversion unit 30, a low pass filter (LPF) 40, and a Manchesterdecoding circuit 50.

The photoelectric conversion unit 30 includes a laser diode (LD) driver32, a laser diode (LD) 34, a photodiode (PD) 36, and a transimpedanceamplifier (TIA) 38. Hereinafter, the laser diode driver 32, the laserdiode 34, the photodiode 36, and the transimpedance amplifier 38 arealso referred to as an LD driver 32, an LD 34, a PD 36, and a TIA 38,respectively. Also, the low pass filter 40 is referred to as an LPF 40.The Manchester decoding circuit 50 includes a single-edge intervalcounter 60 and a decoding unit 70. The Manchester decoding circuit 50 isan example of a decoding circuit.

The DSP 10 receives from the host device a parallel high-speedtransmitting data signal (digital signal) containing information to betransmitted to another optical transceiver 100. The DSP 10 converts thereceived high-speed transmitting data signal to an analog transmittingsignal such as, for example, a PAM (Pulse Amplitude Modulation) 4signal. The DSP 10 outputs the converted analog transmitting signal tothe LD driver 32.

The DSP 10 also receives an analog received signal from the TIA 38, suchas a PAM4 signal converted from an optical signal received from theother optical transceiver 100. The DSP 10 converts the analog receivedsignal into a high-speed received data signal (digital signal) inparallel. The DSP 10 outputs the converted high-speed received datasignal to the host device. The DSP 10 includes a high pass filter (notillustrated). The high pass filter removes monitoring controlinformation superimposed on the analog received signal received from theTIA 38.

For example, the high-speed transmitting data signal and the high-speedreceived data signal are NRZ (Non-Return-to-Zero) signals. Theinterconversion of the high-speed transmitting data signal and thehigh-speed received data signal with the PAM4 signals may take place ina conversion circuit connected between the DSP 10 and, the LD driver 32and TIA 38.

The Manchester encoding circuit 20 encodes monitoring controlinformation (transmission monitoring control information) fortransmission by Manchester encoding and outputs the encoded monitoringcontrol information as a low-speed transmitting data signal formonitoring to the LD driver 32. The transfer rate of the monitoringlow-speed transmitting data signal is lower than the transfer rate ofthe data signal to be transmitted and received by the DSP 10. Forexample, the frequency of the low-speed transmitting data signal outputfrom the Manchester encoding circuit 20 is about tens of kHz to aboutone MHz, whereas the PAM4 signal transmitted to and received from theDSP 10 is about 25G baud.

The LD driver 32 superimposes the low-speed transmitting data signalcontaining monitoring control information on an analog transmittingsignal received from the DSP 10. The LD driver 32 outputs thesuperimposed analog transmitting signal to the LD 34 to drive the LD 34.The LD 34 generates an optical transmitting signal from the analogtransmitting signal (an electrical signal) in which the low-speedtransmitting data signal is superimposed, and outputs the generatedoptical transmitting signal to the optical cable.

For example, the optical signal output from the LD 34 is a PAM4 signal.The superimposition of the low-speed transmitting data signal formonitoring on the analog transmitting signal may be performed bychanging the bias current supplied to the LD driver 32 in response tothe low-speed transmitting data signal.

The PD 36 receives the superimposed optical reception signal fromanother optical transceiver 100 via an optical cable. The PD 36generates a current signal (an electrical signal) from the receivedoptical reception signal and outputs the generated current signal to theTIA 38. For example, the optical signal received by the PD 36 is thePAM4 signal. The TIA 38 amplifies the minute current signal receivedfrom the PD 36 and outputs the amplified current signal as a voltagesignal (analog received data signal) to the DSP 10 and the LPF 40.

The LPF 40 cuts off the high frequency component of the seriallyreceived data signal received from the TIA 38 to extract the low-speedreceived data signals superimposed on the received data signals (themonitoring control information encoded by Manchester encoding). The LPF40 outputs the extracted low-speed received data signal to theManchester decoding circuit 50. The low-speed received data signal is anexample of a Manchester encoded monitoring signal.

The Manchester decoding circuit 50 decodes the Manchester encodedlow-speed received data signal using a single-edge interval counter 60and a decoding unit 70. The Manchester decoding circuit 50 outputs thedecoded low-speed received data signal as reception monitoring controlinformation to the host device.

Examples of the single-edge interval counter 60 and the decoding unit 70are illustrated in FIGS. 6 and 7 . For example, the transmissionmonitoring control information and the reception monitoring controlinformation may include information for controlling the on/off oftransmitting the optical signal, information for setting thetransmitting speed or data rate of the optical signal, information forsetting the intensity of the optical signal, and information forindicating the power voltage or temperature of the optical transceiver100.

[Relationship Between Manchester Code and Edge Interval]

FIG. 2 is a signal waveform diagram illustrating an example of therelationship between Manchester code and an edge interval D. Forexample, when the data generated in synchronization with thesynchronized clock is a logical value 1, the Manchester encoding circuit20 of FIG. 1 assigns a rising edge (code value P) to the midpoint of theclock cycle. When the data generated in synchronization with thesynchronized clock is a logical value 0, the Manchester encoding circuit20 falls to the middle point of the clock cycle and assigns an edge(code value N). The code value P is an example of the first logicalvalue, and the code value N is an example of the second logical value.In Manchester code, the falling edge may be set to the code value P, andthe rising edge may be set to the code value N.

The Manchester encoding circuit 20 then assigns the rising edge orfalling edge to a position corresponding to the edge (falling edge) ofthe synchronized clock in accordance with the edge allocated to themiddle of the clock cycle. In the Manchester code generated in thismanner, when the period of the synchronized clock, that is, the bitwidth is “T”, the edge intervals D of the two rising edges adjacent toeach other are “T”, “1.5 T”, or “2 T”. The edge interval D may be theinterval of the falling edges adjacent to each other. Hereinafter, anexample of the two rising edges D adjacent to each other will bedescribed.

In FIG. 2 , the number indicated by the arc arrow connecting twoconsecutive edge intervals D correspond to the number of the ratio R(n)of the change in edge interval D illustrated in FIGS. 3 and 4 . Of thetwo consecutive edge intervals D, the first edge interval D(n−1) is anexample of a first time interval between the first edge and the secondedge immediately after detecting the first edge. Of the two consecutiveedge intervals D, the later edge interval D(n) is an example of a secondtime interval between the second edge and the third edge immediatelyafter the second edge. Furthermore, the next edge interval D(n+1) of theedge interval D(n) is an example of a third time interval between thethird edge and the fourth edge immediately after detecting the thirdedge. The first, second and third edges are rising edges adjacent toeach other in the signal waveform of the Manchester code. In theManchester code of the lower signal waveform illustrated in FIG. 2 , thecode values P, P, N, and P, which are represented by a single dashedline frame, are the same pattern as the correction pattern inserted atpredetermined intervals. The correction pattern is described in FIG. 8 .

[Change and Ratio of Edge Interval]

FIG. 3 is a diagram illustrating an example of a ratio R(n) representingchanges in two consecutive edge intervals D(n−1) and D(n) in Manchestercode. The ratio R(n) is given by a formula (1).

$\begin{matrix}{{R(n)} = {{D(n)}/{D\left( {n - 1} \right)}}} & (1)\end{matrix}$There are seven different ratios R(n) of the change in edge interval D,as shown in numbers (1) to (7).

If the edge interval D is changed from “2 T” to “T”, then the ratio R(n)of change in edge interval D is 0.5. If the edge interval D changes from“1.5 T” to “T”, then the ratio R(n) of change in edge interval D is0.67. If the edge interval D changes from “2 T” to “1.5 T”, then theratio R(n) of change in edge interval D is 0.75.

If the edge interval D remains unchanged (“T” to “T”, “1.5 T”, to “1.5T” or “2 T” to “2 T”), then the ratio R(n) of the edge interval D is“1”. If the edge interval D changes from “1.5 T” to “2 T”, then theratio R(n) of change in edge interval D is “1.33”. If the edge intervalD changes from “T” to “1.5 T”, then the ratio R(n) of change in edgeinterval D is 1.5. If the edge interval D is changed from “T” to “2 T”,then the ratio R(n) of change in edge interval D is “2”.

[Estimation of Bit Width and Decoding of Code Value]

FIG. 4 is a diagram illustrating an example of a method of estimating anedge interval D according to a ratio of two consecutive edge intervals Dof a low-speed received data signal. The numbers and ratios R(n) shownin parentheses on the left-hand side of FIG. 4 correspond to the numbersand ratios R(n) shown in FIG. 3 . The ratio R(n) is the ratioD(n)/D(n−1) of two successive edge intervals D(n−1), and D(n). The edgeinterval D(n−1) is a preceding edge interval D, and the edge intervalD(n) is a subsequent edge interval D.

In FIG. 4 , the median values of the two adjacent ratios R(n) are set tothresholds VT1, VT2, VT3, VT4, VT5, and VT6, respectively. The thresholdvalue VT0 is set to 0.42, which is obtained by subtracting 0.08 of thedifference between the threshold value VT1=0.58 and the ratio R(n)=0.5,from the ratio R(n)=0.5. The threshold value VT7 is set to a value(2.25) obtained by adding 0.25 that is the difference between the ratioR(n)=2 and the threshold value VT6=1.75, to the ratio R(n)=2. Thethreshold values VT0 to VT7 may be collectively referred to as thresholdvalues VT.

For example, the decoding unit 70 obtains a ratio R(n) from the edgeinterval D(n−1) and the edge interval D(n) that are actually measured.The decoding unit 70 compares the determined ratio R(n) with eachthreshold value VT. The decoding unit 70 estimates how many times (1time, 1.5 times, or 2 times) longer is the subsequent edge interval D(n)among the two edge intervals D(n−1) and D(n) than the bit width T basedon the comparison result. Here, the bit width T is the period of thesynchronized clock used in Manchester encoding. Hereinafter, the ratioof the edge interval D to the bit width T (clock period) is alsoreferred to as the clock number. For example, if the edge interval D is1.5 times longer than the bit width T, the clock number of the edgeinterval D is “1.5”.

By estimating the number of clocks in the edge interval D, a Manchesterencoded code value included in the low speed received data signal LDTcan be decoded, as illustrated in FIG. 5 . In addition, because thenumber of clocks of the edge interval D can be estimated, the code valuecan be decoded even when the period of the synchronized clock used inManchester encoding is unknown or when the period of the synchronizedclock used in Manchester encoding varies. In other words, each time theratio R(n) is obtained, the period of the synchronized clock can beestimated and the period of the synchronized clock can be updated.

For example, when the ratio R(n) is “0.75”, the period of thesynchronized clock, that is, the bit width T, can be estimated bydividing the estimated edge interval D(n) by “1.5”. When the ratio R(n)is “1.33”, then the period of the synchronized clock, that is, the bitwidth T, can be estimated by dividing the estimated edge interval D(n)by “2”. Incidentally, the period of the synchronized clock, that is, thebit width T, may be estimated based on the average value obtained bydividing the sum of the edge interval D(n−1) and D(n) by 2.

In (3) of FIG. 4 , the bit width T may be estimated not by multiplyingthe edge interval D(n) by 1/1.5, but by multiplying the edge intervalD(n−1) by ½. In (6) and (7) of FIG. 4 , the bit width T may estimate theedge interval D(n−1) as the bit width T rather than using the edgeinterval D(n). In this case, the period of the synchronized clock can becalculated without calculation or by shifting the binary bit stringrepresenting the edge interval D(n−1) to the right by one bit, therebyreducing the amount of calculation.

The decoding unit 70 determines that the edge intervals D(n−1) and D(n)are equal to each other when the ratio R (n) is greater than “0.88” andless than or equal to “1.17”. In this case, the edge intervals D(n−1)and D(n) are, for example, any of “T”, “1.5 T”, or “2 T”, and are notdetermined. Therefore, the decoding unit 70 suspends the estimation ofthe edge interval D(n). Even when the estimation of the edge interval Dis suspended, the decoding unit 70 can decode the code value included inthe low speed received data signal LDT using the method described inFIG. 5 . For example, when the edge interval is established in the edgeinterval D(n−1), the edge interval of the edge interval D(n−1) may bemade the edge interval D(n).

When the ratio R(n) is less than or equal to the threshold value VT0,the decoding unit 70 determines that the edge interval D(n) is too smallto be estimated. When the ratio R(n) is greater than the threshold valueVT7, the decoding unit 70 determines that the edge interval D(n) is toolarge to be estimated.

When the decoding unit 70 has determined that estimation is notpossible, the decoding unit 70 performs the acquisition of the ratioR(n) of the edge interval D from the beginning again. In this case, thedecoding unit 70 obtains the ratio R(n+2)=D(n+2)/D(n+1) of the next edgeinterval D(n+1) of the edge interval D(n) and the further next edgeinterval D(n+2). Edge interval D (n+1) is an example of a third timeinterval between the third edge and the fourth edge immediately afterdetecting the third edge. Edge interval D (n+2) is an example of afourth time interval between the fourth edge and the fifth edge detectedimmediately after detecting the fourth edge. The third, fourth, andfifth edges are rising edges adjacent to each other in the signalwaveform of the Manchester code.

In this manner, by setting the upper and lower limit values of the ratioR(n) as the threshold values VT0 and VT7, it is possible to detect anedge erroneously caused by noise or the like, changes in transmissionrate of the Manchester code, or an interruption of transmission of thelow-speed received data signal LDT. In this case, the decoding unit 70causes the single-edge interval counter 60 to start again from themeasurement of the first edge interval D(n−1). Thus, the decoding unit70 can resume decoding of the low speed received data signal LDT basedon the ratio R(n) between the new edge intervals D(n−1) and D(n).

The symbols ERR1=1, BW1=1, BW2=1, BW3=1, BW4=1, BW5=1, BW6=1, BW7=1, andERR2=1 on the right side of FIG. 4 are the values of the signals used inthe decoding unit 70 illustrated in FIG. 7 , and only one of them is setto the logical value 1. The codes ERR1 and ERR2 are error signals, andeach of the codes BW1 to BW7 is a bit width signal. Incidentally, insome cases, each of the bit width signals BW1 to BW7 is collectivelyreferred to as a bit width signal BW.

For example, the bit width signal BW1 of the logical value 1 and the bitwidth signal BW2 of the logical value 1 indicate that the clock numberof the edge interval D(n) is “1”, and indicate that the edge intervalD(n) is approximately equal to the bit width T (clock period). The bitwidth signal BW3 of the logical value 1 and the bit width signal BW6 ofthe logical value 1 indicate that the clock number of the edge intervalD(n) is “1.5”, and indicate that the edge interval D(n) is approximatelyequal to 1.5 times the bit width T (clock period).

The bit width signal BW5 of the logical value 1 and the bit width signalBW7 of the logical value 1 indicate that the number of clocks of theedge interval D(n) is “2”, and indicate that the edge interval D(n) isapproximately equal to twice the bit width T (clock period). Thus, thebit width signal BW indicates the estimated result of how many times theedge interval D(n) is the bit width T of the synchronized clock used inthe Manchester coding.

<Method of Decoding Code Value from Edge Interval>

FIG. 5 is an explanatory diagram illustrating an example of a method fordecoding a code value based on an edge interval D of a low-speedreceived data signal LDT and a code value immediately decoded before.FIG. 5 illustrates a decoding operation by the decoding unit 70 of FIG.1 . In FIG. 5 , the edge interval D illustrates the intervals of tworising edges adjacent to each other in the signal waveform of thelow-speed received data signal LDT. As illustrated in FIG. 2 , the edgeinterval D is either “T”, “1.5 T”, or “2 T”.

The reference code value P indicated by the dashed rectangular frameindicates that the last decoded code value is “P”. The reference codevalue N indicated by the dashed rectangular frame indicates that thelast decoded code value is “N”. The dashed circle indicates the lastcode value among the code values determined by decoding. When the lastdetermined code value is “P”, the last determined code value isexpressed as “LastP”, and when the last determined code value is “N”,the last determined code value is expressed as “LastN”. When thedetermined code value is one, the determined code value is “LastP” or“LastN”. The “LastP” becomes a reference code value P at the time of thenext code value determination. The “LastN” becomes a reference codevalue N at the time of the next code value determination.

When the reference code value is “P” and the edge interval D is “T” (seeFIG. 5(a)), the decoding unit 70 determines that the code value P isreceived. When the reference code value is “N” and the edge interval Dis “T” (see FIG. 5(b)), the decoding unit 70 determines that the codevalue N is received.

When the reference code value is “P” and the edge interval D is “1.5 T”(see FIG. 5(c)), the decoding unit 70 determines that the code value Nis received. Incidentally, the code value N represented by parenthesesfollowing the received code value N is decoded the next time. When thereference code value is “N” and the edge interval D is “1.5 T” (see FIG.5(d)), the decoding unit 70 determines that the code value N and thecode value P are received.

When the reference code value is “P” and the edge interval D is “21”(see FIG. 5(e)), the decoding unit 70 determines that the code value Nand the code value P are received. When the reference code value is “N”and the edge interval D is “21” (see FIG. 5(f)), because the pattern isnot present in the Manchester code, the decoding unit 70 determines thata phase shift has occurred and does not determine the received codevalue. However, the decoding unit 70 can determine the code value at thenext appearing edge interval D by determining that the rising edge ofthe latter half is “LastP”.

As illustrated by the waveform in FIG. 2 , the decoding unit 70 decodesManchester code using two consecutive edge intervals D. For example, thedecoding unit 70 determines one or two code values based on the ratioR(n) between the edge interval D(n−1) and the edge interval D(n), anddetermines one or two code values based on the ratio R(n+1) of the edgeinterval D(n) and the edge interval D(n+1). As described above, of theedge intervals D(n−1) and D(n) used for the determination of the codevalue, the later edge interval D(n) is used for the determination of thefollowing code value. As a result, the decoding unit 70 can sequentiallydetermine consecutive code values of the Manchester code usingconsecutive pairs of two edge intervals D.

[Circuit Configuration of Single-Edge Interval Counter]

FIG. 6 is a circuit diagram illustrating an example of the single-edgeinterval counter 60 of FIG. 1 . The single-edge interval counter 60includes a counter 62 and a register 64. The single-edge intervalcounter 60 is an example of an interval measuring unit that repeatedlymeasures the edge distance D of the rising edges adjacent to each otherof the low-speed received data signal LDT. The single-edge intervalcounter 60 detects the rising edge, but may detect the falling edge.

However, the single-edge interval counter 60 detects only apredetermined rising edge or falling edge.

The counter 62 includes a reset terminal RST, a clock terminal CK forreceiving a clock signal CLK, and an output terminal OUT for outputtinga count value CNT. The reset terminal RST receives a Manchester encodedlow-speed received data signal LDT output from the LPF 40 of FIG. 1 .The counter 62 resets the count value CNT to “0” in synchronization withthe rising edge of the low-speed received data signal LDT and increasesthe count value CNT by “1” in synchronization with the clock signal CLK.

Thus, the counter 62 outputs the time interval from the rising edge ofthe low-speed received data signal LDT at each rising edge as the countvalue CNT. That is, the counter 62 repeatedly measures the edgeintervals D of the two rising edges adjacent to each other as a countvalue CNT.

The register 64 includes a load terminal for receiving a low-speedreceived data signal LDT, an input terminal IN for receiving a countvalue CNT, and an output terminal OUT for outputting a loaded countvalue CNT. The register 64 latches the count value CNT insynchronization with the rising edge of the low-speed received datasignal LDT and outputs the latched count value CNT as the edge intervalD from the output terminal OUT.

The signal waveform illustrated in the lower brackets of FIG. 6indicates operation of the single edge interval counter 60. The verticalstripes of the clock signal CLK and the count value CNT in the waveformdiagram indicate that the frequency of the clock signal CLK issufficiently higher than the frequency of the low-speed received datasignal LDT. For example, as a clock signal CLK, a frequency dividingclock signal may be used that divides the frequency of the clock signalused in the DSP 10 by a predetermined frequency dividing ratio, or otherclock signals may be used.

As illustrated in the signal waveform, two rising edges of the low-speedreceived data signal LDT are required to obtain a single-edge intervalD. Further, in order to decode the reception monitoring controlinformation by the decoding unit 70, two consecutive edge intervals D(for example, D(n−1) and D(n)) are required.

The single-edge interval counter 60 may repeatedly measure the edgeinterval D of the falling edges adjacent to each other of the low-speedreceived data signals LDT. In this case, the counter 62 is reset at thefalling edge of the low-speed received data signal LDT. The register 64latches the count value CNT in synchronization with the falling edge ofthe low-speed received data signal LDT and outputs the latched countvalue CNT as the edge interval D from the output terminal OUT. However,the configuration and operation of the decoding unit 70 when measuringthe edge distance D of the falling edge are different from theconfiguration and operation of the decoding unit 70 described below.

[Circuit Configuration of Decoding Unit]FIG. 7 is a circuit diagramshowing an example of the decoding unit 70 of FIG. 1 . The decoding unit70 includes a clock number estimating unit 80 and a code valuedetermining unit 90. The clock number estimating unit 80 includes aprevious value memory 82, eight multipliers 84, eight comparators 86,eight inverters IV, and seven AND circuits. The number of themultipliers 84, the comparators 86, the inverters IV, and the ANDcircuits AND are set according to the number (type) of the ratio R(n)shown in FIG. 3 .

The previous value memory 82 captures the edge interval D(n) receivedfrom the single-edge interval counter 60 in synchronization with therising edge of the low-speed received data signal LDT received at theclock terminal CK and outputs the edge interval D(n) as an edge intervalD(n−1). The edge interval D(n−1) output from the previous value memory82 is supplied to each multiplier 84.

The edge interval D(n) input to the previous value memory 82 is the edgeinterval D of the two rising edges on the slower side of the threerising edges of the low-speed received data signal LDT, as illustratedin the signal waveform of FIG. 6 . That is, the edge interval D(n) is asubsequent edge interval D of the two edge intervals D(n−1) and D(n).

The edge interval D(n−1) output from the previous value memory 82 is theedge interval D of the two rising edges on the earlier side of the threerising edges of the low-speed received data signal LDT, as illustratedin the signal waveform of FIG. 6 . That is, the edge interval D(n−1) isthe prior edge interval D of the two consecutive edge intervals D(n−1)and D(n). The previous value memory 82 is an example of a storage unitthat stores the two consecutive edge intervals D(n−1) and D(n) of thelow-speed received data signal LDT.

Each of the multipliers 84 multiplies the edge interval D(n−1) bydifferent multipliers and outputs the multiplication results to thecomparator 86. The multipliers used in the eight multipliers 84 are“0.42”, “0.58”, “0.71”, “0.86”, “1.17”, “1.42”, “1.752, and “2.25”, andthe threshold values VT0, VT1, . . . , VT7 described in FIG. 4 .

Each of the comparators 86 compares the multiplication result outputfrom the corresponding multiplier 84 with the edge interval D(n), andoutputs the comparison result. Each of the comparators 86 outputs thelogical value 1 when the edge interval D(n) is larger than themultiplication result, and outputs the logical value 0 when the edgeinterval D(n) is smaller than the multiplication result.

The circuit illustrated in FIG. 7 is configured by using a formula (2),which is a modification of the formula (1) described in FIG. 3 .

$\begin{matrix}{{D(n)} = {{R(n)} \cdot {D\left( {n - 1} \right)}}} & (2)\end{matrix}$

Each of the multipliers 84 calculates the right-hand side of the formula(2), and the comparator 86 compares the left-hand side and theright-hand side of the formula (2). That is, the comparator 86 comparesthe edge interval D(n) with the threshold value R(n)•D(n−1).

Thus, the ratio R(n) and the threshold VT can be compared by multiplyingthe edge interval D(n−1) and each threshold VT without performing thedivision shown in the formula (1). As a result, the circuit size of thedecoding unit 70 can be inhibited, and the cost of the opticaltransceiver 100 can be reduced. The circuit configuration of the clocknumber estimating unit 80 is not limited to FIG. 7 .

The threshold R(n)⋅D(n−1) varies depending on the edge interval D(n−1).That is, the clock number estimating unit 80 can update the thresholdvalue each time the previous edge interval D(n−1) is measured, and cancompare the threshold value with the edge interval D(n). Accordingly,even when the frequency of the synchronized clock used in Manchesterencoding changes, or when a synchronized clock jitter or the likeoccurs, the threshold value can be updated following the change infrequency, jitter, or the like. As a result, the accuracy of decoding bythe Manchester decoding circuit 50 can be improved compared to when thethreshold is fixed, and the reliability of the optical transceiver 100can be improved.

Each of the AND circuits AND receives a logical value output from thecorresponding comparator 86 and a logical value inverted by an inverterIV after being output from the right side comparator 86 of thecorresponding comparator 86 by being generated by the right sidecomparator by inputting a signal after multiplying a multiplying factorgreater than the multiplying factor of the corresponding comparator 86by one. Each AND circuit AND outputs the logic operation result as thebit width signal BW (BW1, BW2, BW7) to the code value determining unit90.

The logical value output from the leftmost-side comparator 86 of FIG. 7is inverted by the inverter IV and output to the code value determiningunit 90 as the error signal ERR1. The logical value output from thecomparator 86 on the rightmost side of FIG. 7 is output to the codevalue determining unit 90 as the error signal ERR2.

For example, when the edge interval D(n−1) and D(n) are both “1”, thecomparators 86 corresponding to four multipliers 84 having multiplyingfactors from 0.42-fold to 0.88-fold outputs a logical value of 1, andthe comparator 86 corresponding to four multipliers 84 havingmultiplying factors from 1.17-fold to 2.25-fold outputs a logical valueof 0. In this case, the clock number estimating unit 80 outputs the bitwidth signal BW4 of the logical value 1, the bit width signals BW1 toBW3 and BW5 to BW7 of the logical value 0, and the error signals ERR1and ERR2. That is, the bit width signal BW4 of the logical value 1indicates that the ratio R(n) is greater than 0.88 and less than orequal to 1.17.

Also, when the edge interval D(n−1) is “1” and the edge interval D(n) is“2”, the comparators 86 corresponding to seven multipliers 84 of0.42-fold to 1.75-fold output a logical value 1, and the comparator 86corresponding to 2.25-fold multiplier 84 outputs a logical value 0. Inthis case, the clock number estimating unit 80 outputs the bit widthsignal BW7 of the logical value 1, the bit width signals BW1 to BW6 ofthe logical value 0, and the error signals ERR1 and ERR2. That is, thebit width signal BW7 of the logical value 1 indicates that the ratioR(n) is greater than 1.75 and less than or equal to 2.25.

When the edge interval D(n−1) is “1” and the edge interval D(n) is “3”,then all of the comparators 86 output a logical value 1. In this case,the clock number estimating unit 80 outputs the error signal ERR1 of thelogical value 1, the bit width signals BW1 to BW7 of the logical value0, and the error signal ERR2. That is, the error signal ERR1 of thelogical value 1 indicates that the ratio R(n) is less than or equal to0.42.

When the edge interval D(n−1) is “3” and the edge interval D(n) is “1”,then all of the comparators 86 output a logical value 0. In this case,the clock number estimating unit 80 outputs the error signal ERR2 of thelogical value 1, the error signal ERR1 of the logical value 0, and thebit width signals BW1 to BW7. That is, the error signal ERR2 of logicalvalue 1 indicates that the ratio R(n) is greater than 2.25.

The code value determining unit 90 includes a code value extracting unit92 and a last value retaining unit 94. The code value extracting unit 92extracts the code value included in the received low-speed received datasignal LDT based on the logical value of the bit width signals BW1 toBW7 output from the clock number estimating unit 80 and the code value(P or N) retained by the last value retaining unit 94. The code valueretained by the last value retaining unit 94 is the code value extractedby the code value extracting unit 92 immediately before.

Extraction of the code value by the code value extracting unit 92 isperformed according to the method for decoding the code valueillustrated in FIG. 5 . The code value decoded immediately before thestate shown by the dashed line rectangular frame in FIG. 5 is retainedin the last value retaining unit 94 and is used to extract the codevalue in the code value extracting unit 92. The LastP or LastNillustrated in FIG. 5 by a dashed line circle is the last code valueextracted by the code value extracting unit 92 and is retained in thelast value retaining unit 94. The decoding unit 70 outputs the codevalue extracted by the code value extracting unit 92 as the receptionmonitoring control information to the host device.

The decoding unit 70 illustrated in FIG. 7 can decode the Manchesterencoded code value by comparing the ratio R(n) between the twoconsecutive edge intervals D(n−1) and D(n) with the threshold value VTin the low-speed received data signal LDT. In other words, the clocknumber estimating unit 80 can obtain the clock number of the edgeinterval D(n) by comparing the threshold value R(n)⋅D(n−1) updated eachtime the previously measured edge interval D(n−1) is updated with theedge interval D(n).

Generally, when information is encoded by Manchester encoding, asynchronization bit string such as a preamble is inserted at any timing.Insertion of the synchronization bit string prevents only many codevalues P or only many code values N from being consecutive.

For this reason, the Manchester decoding circuit 50 can successivelyestimate the number of clocks of the edge interval D by continuing toreceive the low-speed received data signal LDT and sequentially decodethe Manchester code. That is, once the low-speed received data signalLDT is successfully decoded, decoding of the low-speed received datasignal LDT can be continued using the method illustrated in FIG. 5 .

[Manchester Decoding Circuit State Transition Diagram]

FIG. 8 is a state transition diagram illustrating an example ofoperation of the Manchester decoding circuit 50 of FIG. 1 . TheManchester decoding circuit 50 has a start state, a measurement state, aLastP state, and a LastN state. In the LastP state, the decoding unit 70illustrated in FIG. 7 determines the code value according to the methodfor determining the reference code value P illustrated in FIG. 5 . Inthe LastN state, the decoding unit 70 determines the code valueaccording to the method for determining the reference code value Nillustrated in FIG. 5 .

First, in the start state, the single-edge interval counter 60 startsreceiving the Manchester encoded low-speed received data signal LDT.When the first rising edge is detected, the single-edge interval counter60 changes to the measurement state of the edge interval D(n−1) (seeFIG. 8(a)). When the first edge interval D(n−1) fails to be measured,the single-edge interval counter 60 resets the count value CNT andremeasures the edge interval D(n−1) (see FIG. 8(b)).

When the single-edge interval counter 60 succeeds in measuring the firstedge interval D(n−1) by detecting the next rise of thelow-speed-receiving data signal LDT, the single-edge interval counter 60determines the edge interval D(n−1) and transits to the LastP state (seeFIG. 8(c)). The last value retaining unit 94 retains the code value P asthe last value.

As illustrated in FIG. 2 , when the code value is “P”, the rising edgeof the signal is positioned at the middle point of the bit width T (thefalling edge of the synchronized clock). When the code value is “N”, thefalling edge of the signal is positioned at the middle point of the bitwidth T (the falling edge of the synchronized clock). When the risingedge of the signal is within the synchronized clock, the rising edge ofthe signal is located at an end point (the rising edge of thesynchronized clock) of bit width T.

Therefore, when the edge interval D(n−1) is measured for the first timeafter the transition to the measurement state, either the LastP state orthe LastN state can be taken. In this embodiment, when the edge intervalD(n−1) is measured for the first time after the transition to themeasurement state, the state is moved to the LastP state as a tentativestate. When the edge interval D(n−1) is measured for the first timeafter the transition to the measurement state, the state of theManchester decoding circuit 50 may be moved to the LastN state.

In the LastP state or the LastN state, the single-edge interval counter60 stores the edge interval D(n−1) in the previous value memory 82 whenthe next rise of the low-speed received data signal LDT is detected. Asdescribed in FIG. 7 , the clock number estimating unit 80 sets eitherthe bit width signals BW1 to BW7 or the error signals ERR1 and ERR2 tothe logical value 1 based on the ratio R(n) (=edge intervalD(n)/D(n−1)). That is, receiving the Manchester code starts.

In the LastP state, when the bit width signal BW1 having the logicalvalue 1 representing the edge interval D(n)=“T” or the bit width signalBW2 having the logical value 1 is received, the code value extractingunit 92 determines the reception of the code value P and remains in theLastP state (see FIG. 8(d)). The last value retaining unit 94 retainsthe code value P determined by the code value extracting unit 92 as thelast value.

In the LastP state, when the bit width signal BW3 of the logical value 1representing the edge interval D(n)=“1.5 T” or the bit width signal BW6of the logical value 1 is received, the code value extracting unit 92determines the reception of the code value N and changes the state tothe LastN state (see FIG. 8(e)). The last value retaining unit 94retains the code value N determined by the code value extracting unit 92as the last value.

In the LastP state, when receiving the bit width signal BW5 of thelogical value 1 representing the edge interval D(n)=“2 T” or the bitwidth signal BW7 of the logical value 1, the code value extracting unit92 determines the reception of the code values N and P and remains inthe LastP state (see FIG. 8(f)). The last value retaining unit 94retains the last determined code value P determined by the code valueextracting unit 92 as the Last value.

In the LastP state, when receiving the error signal ERR1 of the logicalvalue 1 or the error signal ERR2 of the logical value 1, the code valuedetermining unit 90 stops operation of repeatedly measuring the edgeinterval D(n). Then, the code value determining unit 90 transits to themeasurement state in order to start the measurement from the first edgeinterval D(n−1) (see FIG. 8(g)).

The error signal ERR1 of the logical value 1 is output when the edgeinterval D(n) is too short. The error signal ERR2 of the logical value 1is output when the edge interval D(n) is too long. This allows theManchester decoding circuit 50 to resume decoding of the Manchester codeagain without continuing to detect incorrect code values.

On the other hand, in the LastN state, when receiving the bit widthsignal BW1 having the logical value 1 representing the edge intervalD(n)=“T”, or the bit width signal BW2 having the logical value 1, thecode value extracting unit 92 determines the reception of the code valueN and stays in the LastN state (see FIG. 8(h)). The last value retainingunit 94 retains the code value N determined by the code value extractingunit 92 as the last value.

In the LastN state, when receiving the bit width signal BW3 of thelogical value 1 representing the edge interval D(n)=“1.5 T” or the bitwidth signal BW6 of the logical value 1, the code value extracting unit92 determines the reception of the code values N and P, and changes thestate to the LastP state (see FIG. 8(i)). The last value retaining unit94 retains the last determined code value P as the last value determinedby the last code value extracting unit 92.

In the LastN state, when receiving the bit width signal BW5 of thelogical value 1 representing the edge interval D(n)=“2 T”, or the bitwidth signal BW7 of the logical value 1, the code value extracting unit92 determines that the measurement of the edge interval D(n) waserroneous. The code value extracting unit 92 transits to the LastP statewithout performing a receiving determination of the code value (see FIG.8(j)).

In the LastN state, when receiving the error signal ERR1 of the logicalvalue 1 or the error signal ERR2 of the logical value 1, the code valuedetermining unit 90 stops the operation of repeatedly measuring the edgeinterval D(n). Then, the code value determining unit 90 transits to themeasurement state in order to start the measurement from the first edgeinterval D(n−1) (see FIG. 8(k)). This allows the Manchester decodingcircuit 50 to resume decoding of Manchester codes again withoutcontinuing to detect incorrect code values.

In the LastP state or the LastN state, when the bit width signal BW4 isa logical value 1, the code value extracting unit 92 of the code valuedetermining unit 90 suspends the estimation of the edge interval D(n).However, the decoding unit 70 can decode the Manchester code using themethod illustrated in FIG. 5 even when the estimation of the edgeinterval D(n) is suspended. For example, when the edge interval isestablished in the edge interval D(n−1), the edge interval of the edgeinterval D(n−1) may be made the edge interval D(n).

In addition, when the correction pattern (codes P, P, N, and P)illustrated in FIG. 2 is inserted into the bit string of the Manchestercode, the decoding unit 70 detects the edge interval D(n−1) and D(n)having the ratio R (n) of “2”. The decoding unit 70 determines thereception of the code value N by outputting the bit width signal BW7having the logical value 1. Accordingly, the decoding unit 70 canestimate the edge interval D(n) by receiving the correction pattern evenwhen all the bits of the code value P other than the correction patternare received consecutively and the estimation is suspended by the sameedge intervals D(n−1) and D(n).

The decoding unit 70 continues to falsely detect consecutive code valuesP as consecutive code values N, and even when the LastN state isrepeated, the decoding unit 70 changes its state to the LastP state uponreceiving the correction pattern. Accordingly, the receiving operationof the code value can be restored to normal, and then the correct codevalue can be detected.

Thus, in the first embodiment, the optical transceiver 100 can decodethe Manchester encoded monitoring signal even when the period of thesynchronized clock used in Manchester encoding is unknown or when theperiod of the synchronized clock varies. Specifically, the opticaltransceiver 100 can decode a Manchester encoded monitoring signal basedon a ratio between two consecutive edge intervals D(n−1) and D(n).

That is, the optical transceiver 100 can decode the Manchester encodedmonitoring information, no matter how the period of the synchronizedclock used in Manchester encoding is set. Thus, a common opticaltransceiver 100 can be used to decode various Manchester codes withperiods of synchronized clocks used in various host devices.

This reduces the development cost, manufacturing cost, and maintenancecost of the optical transceiver 100 as compared to the case where theoptical transceiver 100 is prepared for each system. For example,monitoring information is communicated redundantly to signals that areoriginally communicated. For this reason, the period of the synchronizedclock used in Manchester encoding may be changed depending on theinstallation environment and operating environment of the system inorder to avoid the influence on the communication quality of the PAM4signal, which is the original communication target, and the like. Again,Manchester encoded monitoring control information can be automaticallydecoded following changes in period of the synchronized clock.

According to the present disclosure, an optical transceiver capable ofdecoding a code value of Manchester code can be provided even when aperiod of a synchronized clock used in Manchester encoding is unknown orwhen the period of the synchronized clock varies.

As discussed above, although embodiments of the present disclosure havebeen described, the present disclosure is not limited to theabove-described embodiments. Various alternations, modifications,substitutions, additions, deletions, and combinations are possiblewithin the scope of the claims. They are definitely within the technicalscope of the present disclosure.

What is claimed is:
 1. An optical transceiver configured to receive anoptical signal on which a monitoring signal encoded by Manchesterencoding is superimposed, the optical transceiver comprising: a decodingcircuit configured to decode the monitoring signal from an electricalsignal generated from the optical signal, the decoding circuitincluding: an interval measuring unit configured to detect only a risingedge or a falling edge of a waveform of the electrical signal, tomeasure a first time interval between a detected first edge and a secondedge detected immediately after detecting the first edge, and to measurea second time interval between the second edge and a third edge detectedimmediately after detecting the second edge, and a decoding unitconfigured to decode the monitoring signal encoded by the Manchesterencoding based on a ratio between the first time interval and the secondtime interval; a clock number estimating unit configured to estimate anumber of clocks of the second time interval with respect to a period ofa synchronized clock used in the Manchester encoding of the monitoringsignal based on the ratio between the first time interval and the secondtime interval; and a code value determining unit configured to decodethe monitoring signal encoded by the Manchester encoding based on theestimated number of clocks of the second time interval and a logicalvalue of the monitoring signal decoded immediately before.
 2. Theoptical transceiver as claimed in claim 1, wherein the clock numberestimating unit comprises; a storage unit configured to store the firsttime interval, a plurality of multipliers multiplying the first timeinterval by each of a plurality of thresholds, and a plurality ofcomparators configured to compare each of a plurality of multiplicationresults from the plurality of multipliers to the second time interval,wherein the clock number estimating unit is configured to estimate thenumber of clocks based on a plurality of comparative results from theplurality of comparators.
 3. The optical transceiver as claimed in claim1, wherein the interval measuring unit further measures a third timeinterval between the third edge and a fourth edge detected immediatelyafter detecting the third edge, and wherein the decoding unit isconfigured to decode a first logical value corresponding to the thirdedge of the monitoring signal based on the ratio between the first timeinterval and the second time interval and to decode a second logicalvalue corresponding to the fourth edge of the monitoring signal based onthe ratio between the second time interval and the third time interval.4. The optical transceiver as claimed in claim 1, wherein, upondetecting by the decoding unit that the ratio between the first timeinterval and the second time interval is out of a predetermined range,the interval measuring unit further measures a third time intervalbetween the third edge and a fourth edge detected immediately afterdetecting the third edge, and a fourth time interval between the fourthedge and a fifth edge detected immediately after detecting the fourthedge, and wherein the decoding unit decodes the monitoring signal basedon the ratio between the third time interval and the fourth timeinterval.
 5. The optical transceiver as claimed in claim 1, wherein themonitoring signal encoded by the Manchester encoding and received by thedecoding circuit includes a correction pattern in which a first logicalvalue, the first logical value, a second logical value, and the firstlogical value sequentially appear, and wherein the first logical valueis indicated by a rising edge or a falling edge detected by the intervalmeasuring unit.
 6. A method for controlling an optical transceiverconfigured to receive an optical signal on which a monitoring signalencoded by Manchester encoding is superimposed and to decode themonitoring signal from an electrical signal generated from the opticalsignal, the method comprising steps of: detecting only a rising edge ora falling edge of a waveform of the electrical signal; measuring a firsttime interval between a detected first edge and a second edge detectedimmediately after detecting the first edge; measuring a second timeinterval between the second edge and a third edge detected immediatelyafter detecting the second edge; estimating a number of clocks of thesecond time interval with respect to a period of a synchronized clockused in the Manchester encoding of the monitoring signal based on theratio between the first time interval and the second time interval; anddecoding the monitoring signal encoded by the Manchester encoding basedon a ratio between the first time interval and the second time interval,the estimated number of clocks of the second time interval and a logicalvalue of the monitoring signal decoded immediately before.
 7. An opticaltransceiver configured to receive an optical signal on which amonitoring signal encoded by Manchester encoding is superimposed, theoptical transceiver comprising: a decoding circuit configured to decodethe monitoring signal from an electrical signal generated from theoptical signal, the decoding circuit including: an interval measuringunit configured to detect only a rising edge or a falling edge of awaveform of the electrical signal, to measure a first time intervalbetween a detected first edge and a second edge detected immediatelyafter detecting the first edge, and to measure a second time intervalbetween the second edge and a third edge detected immediately afterdetecting the second edge; and a decoding unit configured to decode themonitoring signal encoded by the Manchester encoding based on a ratiobetween the first time interval and the second time interval, whereinthe monitoring signal encoded by the Manchester encoding and received bythe decoding circuit includes a correction pattern in which a firstlogical value, the first logical value, a second logical value, and thefirst logical value sequentially appear, and wherein the first logicalvalue is indicated by a rising edge or a falling edge detected by theinterval measuring unit.
 8. The optical transceiver as claimed in claim7, wherein the decoding unit comprises: a clock number estimating unitconfigured to estimate a number of clocks of the second time intervalwith respect to a period of a synchronized clock used in the Manchesterencoding of the monitoring signal based on the ratio between the firsttime interval and the second time interval; and a code value determiningunit configured to decode the monitoring signal encoded by theManchester encoding based on the estimated number of clocks of thesecond time interval and a logical value of the monitoring signaldecoded immediately before.
 9. The optical transceiver as claimed inclaim 8, wherein the clock number estimating unit comprises: a storageunit configured to store the first time interval; a plurality ofmultipliers multiplying the first time interval by each of a pluralityof thresholds, and a plurality of comparators configured to compare eachof a plurality of multiplication results from the plurality ofmultipliers to the second time interval, wherein the clock numberestimating unit is configured to estimate the number of clocks based ona plurality of comparative results from the plurality of comparators.10. The optical transceiver as claimed in claim 7, wherein the intervalmeasuring unit further measures a third time interval between the thirdedge and a fourth edge detected immediately after detecting the thirdedge, and wherein the decoding unit is configured to decode a firstlogical value corresponding to the third edge of the monitoring signalbased on the ratio between the first time interval and the second timeinterval and to decode a second logical value corresponding to thefourth edge of the monitoring signal based on the ratio between thesecond time interval and the third time interval.
 11. The opticaltransceiver as claimed in claim 7, wherein, upon detecting by thedecoding unit that the ratio between the first time interval and thesecond time interval is out of a predetermined range, the intervalmeasuring unit further measures a third time interval between the thirdedge and a fourth edge detected immediately after detecting the thirdedge, and a fourth time interval between the fourth edge and a fifthedge detected immediately after detecting the fourth edge, and whereinthe decoding unit decodes the monitoring signal based on the ratiobetween the third time interval and the fourth time interval.